The present invention relates to corona reactors, and more particularly, to a plasma reactor of the dielectric barrier discharge type and its use in plasma-based gas and liquid purification.
Plasma may be defined as an electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized. It is sometimes referred to as the fourth state of matter, distinct from the solid, liquid and gaseous states.
When energy, such as heat, is continuously applied to a solid, it first melts, then it vaporizes and finally electrons are removed from some of the neutral gas atoms and molecules to yield a mixture of positively charged ions and negatively charged electrons, while overall neutral charge density is maintained. When a significant portion of the gas has been ionized, its properties will be altered so substantially that little resemblance to solids, liquids and gases remains. A plasma is unique in the way in which it interacts with itself, with electric and magnetic fields and with its environment. A plasma can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized simultaneously with other electrons recombining with ions to form neutral particles, while photons are continuously being produced and absorbed.
Plasma may be produced in a discharge tube, which is a closed insulating vessel containing a gas at low pressure through which an electric current flows when sufficient voltage is applied to its electrodes.
Normally, air consists of neutral molecules of nitrogen, oxygen and other gases, in which electrons are tightly hound to atomic nuclei. On application of an electric field above a threshold level, some of the negatively charged electrons are separated from their host atoms, leaving them with a positive charge. The negatively charged electrons and the positively charged ions are then free to move separately under the influence of the applied voltage. Their movement constitutes an electric current. This ability to conduct electrical current is one of the more important properties of plasma
Plasma has been widely studied, different technologies have been developed to obtain different types of plasma and industrial applications have emerged.
The use of plasma as an inducer of chemical reactions and its application for treating gaseous, fluid pollutants and biological contaminants has been widely known for the past couple of decades. The catalyzing performance of plasma depends on its characteristics, which in turn depend on the type of discharge. The discharge itself depends on the shape of electrodes, on the nature of the inter-electrode region, on the voltage and current waveforms used for producing the plasma.
There are four known types of plasma production:
1. Electron beam.
2. Pulsed corona discharge.
3. Surface discharge.
4. Silent discharge (dielectric barrier corona discharge).
Treatment of air streams by dielectric barrier corona discharge is being developed as a cost effective and environmentally friendly alternative to conventional methods of air purification against a wide range of chemical and biological contaminants. Controlled reduction of the contaminant content is achieved by varying the discharge power and the contact time.
An electrical discharge is the passage of electrical current through a material that does not normally conduct electricity, such as air. On application of a high voltage source, the normally insulating air is transformed into a conductor, a process called electrical breakdown, and sparks, which are a form of electrical discharge, fly.
There are several types of electrical discharges:
1. The corona, which is a xe2x80x98partialxe2x80x99 discharge occurring when a highly heterogeneous electric field is imposed. Typically, a very high electric field is present adjacent to a sharp electrode, and a net production of new electron-ion pairs occurs in this vicinity. The corona typically has a very low current and very high voltage.
2. The glow discharge, which typically has a voltage of several hundred volts, and currents up to 1 Amp. A small electron current is emitted from the cathode by collisions of ions, excited atoms and photons, and then multiplied by successive electron impact ionization collisions in the cathode fall region.
3. The arc discharge, which is a high current, low voltage discharge, in which electron emission from the cathode is produced by thermionic and/or field emission.
Gas phase corona reactor (GPCR) technology enables the use of electrical discharges in order to accelerate (heat up) electrons to very high energies, while the rest of the gas stays at room temperature. The energized electrons attack background gas molecules producing highly reactive radicals such as [O], [OH], [N], etc., which in turn decompose various air contaminants.
Volatile organic compounds (VOCs) are an example of common air pollutants released in a number of industrial processes. Emission of VOCs is conventionally controlled by techniques such as thermal oxidation, catalytic oxidation, activated carbon adsorption, bio-filtration, etc. These technologies are generally expensive and have high energy requirements. Growing world concern for environmental protection has promoted testing and evaluation of a number of alternate techniques for abatement of VOCs.
Non-thermal plasma generated by GPCRs has developed as a cost effective and environmentally friendly method for destroying VOCs. The majority of the electrical energy applied to the reactor goes into the production of energetic electrons rather than into producing ions and heating the ambient gas, which is a more efficient and cost-effective method of decomposing toxic compounds than conventional methods.
Non-thermal plasma is highly effective in promoting oxidation, enhancing molecular dissociation and producing free radicals that cause the enhancement of chemical reactions, thereby converting pollutants to harmless by-products.
GPCRs of the dielectric barrier discharge (DBD) type have historically been used to produce industrial quantities of ozone, which have been used in the air and water purification fields. In ozone-based air purification, contaminated fluid is brought into contact with ozone (produced by various methods) while in plasma-based air purification the contaminated fluid is driven through a corona reactor and exposed to plasma. Plasma purification has the advantage of being able to treat extremely difficult compounds such as perfluorocarbons. Plasma purification is also more efficient than ozone purification, providing removal of a significantly greater weight of contaminant per unit energy input.
The conventional design of DBD utilizes a 2-electrode system (grounded tube and inner conducting wire) wherein one or both of the electrodes are covered by an insulating layer preventing arcing across the capacitive barrier by the charge build up. Most of the energized electrons are generated in close proximity to the wire resulting in a small effective plasma volume.
A major factor determining efficiency of a plasma based gas purification device is the structure of the gas flow through the electrodes. The most effective way of increasing efficiency is to lengthen the residence time of the fluid flow within the space between the electrodes in which the electrical discharge occurs. Increasing the time during which the discharge is able to act upon the fluid results in increased detoxification of the fluid, thus improving the quality of purification.
Various methods have been described for lengthening residence time of a gas in an ozone generator. U.S. Pat. No. 5,518,698 to Karlson et al describes an ozone generator in which the resident time for the gas within the generator is increased by lengthening the route for the movement of gas flow between electrodes which are shaped as two coaxial cylinders. The gas is introduced into the annular passageway between the electrodes at an angle so that it swirls in a cyclonic flow path as it travels from one end of the passageway to the other, thereby lengthening the path along which the generated ozone acts upon the gas.
U.S. Pat. No. 5,855,856 to Karlson describes an ozone generator having two concentric electrodes, a vortex chamber installed in front of the ozone generator entrance, with an annular clearance between the electrodes serving as the outlet from the chamber.
In the above designs, the gas flow rate through the ozonizer is limited by the size of the annular clearance between the electrodes, which reduces the amount of treatment the gas receives. The structure of the gas flow described in these designs features low turbulence, which does riot enable the layers in the gas flow to intermix effectively, thereby decreasing the effectiveness of the gas treatment by the discharge-generated ozone.
U.S. Pat. No. 6,027,701 to Ishioka et al. describes an ozone generator which includes a block of electrodes arranged in several rows placed in sequence one after the other. The gas is acted upon by the ozone as it passes through clearances between the electrodes. In this design the high velocity of the gas flow in the entrance chamber of the ozoniser results in a relatively short residence time.
In some plasma generators, a high-voltage electric field is passed through a packed bed of dielectric pellets to form non-thermal plasma in the void spaces between the pellets. The pellets serve to increase the residence time of contaminants in the reactor. These pellets create a high resistance to the gas flow, resulting in a substantial overall pressure drop, necessitating the use of a high power blower and requiring the reactor chamber to be of relatively large dimensions.
U.S. Pat. No. 5,637,198 to Breault describes a volatile organic compound reduction apparatus comprising a reactor-efficient coronal discharge zone and at least one pair of high-dielectric coated electrodes. However, in this system the electrodes are spaced sufficiently far apart to enable untreated compound to pass through areas of minimum energy density between electrodes.
Therefore it would be desirable to provide a dielectric barrier device for efficiently removing a wide range of contaminants from a fluid, in which energy density, effective plasma volume, and residence time of contaminants in the reactor are high, and in which exposure of the fluid to the electrodes in the reactor is homogeneous.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art and provide a dielectric barrier discharge device for converting pollutants in a fluid stream to harmless by-products, wherein electrical discharge is homogeneously distributed within the device. The system is designed to achieve maximum exposure of contaminants to the electrodes of the device, and contaminants have a high residence time within the reactor.
According to a preferred embodiment, there is provided a system for detoxification of contaminated fluids by use of non-thermal plasma produced by dielectric gas phase corona discharge. The system comprises a housing, a corona discharge reactor and an air swiveling device. The reactor comprises upper and lower frame elements, each having a conducting and non-conducting portion and a plurality of cylindrical electrodes. The electrodes are arranged in rows of alternating polarity, so as to form a series of triangular modules, such that the spacing between adjacent electrodes is less than or equal to the diameter of an individual electrode. Each electrode consists of a conducting element surrounded by an insulating jacket. The fluid swiveling device facilitates prolonged exposure of the contaminated fluid to the reactor. When an electrical power supply is connected to the electrodes, a substantially uniform electrical discharge is produced, which reacts with the constituents of the fluid to produce activated radicals. The fluid swiveling device provides effective mixing between activated radicals and fluid, such that toxins and biological contaminants contained in the fluid are attacked and decomposed by the radicals.
A feature of the present invention is the provision of a dielectric barrier discharge device in which the electrical discharge is homogenous and in which exposure time of a fluid to the electric field, and of radicals to the fluid, is high.
An advantage of the present invention is that exposure of contaminants to the areas proximate the electrodes, which have the highest energy density, is maximized.
A further advantage of the present invention is that residence time within the reactor is increased.
A further advantage of the present invention is that energy density within the reactor is high.
A further advantage of the present invention is that a wide range of chemical and biological contaminants can be treated.
A further advantage of the present invention is that cooling can be achieved by passage of oil through the electrode.
A further advantage of the present invention is that arcing is prevented by presence of oil surrounding regions of electrical connections.
A further advantage of the present invention is that a greater weight of contaminant can be removed per unit energy input compared to other known methods.
A further advantage of the present invention is that high temperatures are not required therefore enabling rapid start-up and low maintenance costs.
A further advantage of the present invention is that it is cost-effective and environmentally friendly.
Additional features and advantages of the invention will become apparent from the following drawings and description.